Single phase motor driving circuit and single phase motor driving method

ABSTRACT

A single-phase motor driving circuit includes a controller for outputting a control signal to drive a single-phase motor in a period in which a saw-tooth voltage of a predetermined cycle is larger than a duty setting voltage based on a result of comparing a detected temperature voltage changed based on a temperature detected by a temperature detecting device with a starting duty setting voltage increased with time while a voltage smaller than the detected temperature voltage at the time of starting the single-phase motor is set as an initial value. The controller sets the duty setting voltage as the starting duty setting voltage when the result of the comparison shows that the starting duty setting voltage is smaller than the detected temperature voltage, and the duty setting voltage as the detected temperature voltage when the starting duty setting voltage is larger than the detected temperature voltage.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by any one of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

RELATED APPLICATION INFORMATION

The present application claims priority upon Japanese Patent ApplicationNo. 2002-317597 filed on Oct. 31, 2002, which is herein incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a single-phase motor driving circuit,and a single-phase motor driving method.

2. Description of Related Art

For example, in a motor driving system disclosed in Japanese UtilityModel Application Laid-Open Publication No. Hei3-74199, a comparatoroutputs a pulse signal (control signal) of “H” for driving a motor basedon a saw-tooth wave voltage generated in accordance with a phaseswitching signal. A detected temperature voltage based on a temperaturedetected by a thermistor is inputted to the comparator as a comparisoninput relative to the saw-tooth wave voltage. That is, the comparatoroutputs the pulse signal of “H” in a period in which the saw-tooth wavevoltage is larger than the detected temperature voltage.

The detected temperature voltage fluctuates in a certain range inaccordance with a change in temperature detected by the thermistor.Thus, a width of the pulse signal of “H” outputted by the comparator iswidened or narrowed in accordance with the change in the temperaturedetected by the thermistor. That is, the widening/narrowing of the widthof the pulse signal can vary a revolution speed of the motor.

If a minimum value of the detected temperature voltage is larger thanthat of the saw-tooth wave voltage, a period in which an output of thecomparator always becomes “L” is inevitably generated irrespective ofthe change in temperature detected by the thermistor, as shown in FIG. 4of Japanese Utility Model Application Laid-Open Publication No.Hei3-74199. Consequently, the motor cannot be revolved at a full speed.

A particular situation is now assumed in which a temperature detected bythe thermistor is low to increase a detected temperature voltage, and awidth of a pulse signal of “H” outputted by the comparator becomes thenarrowest. In this case, a maximum value of a detected temperaturevoltage is set large so that a revolution speed of the single-phasemotor can be reduced more, i.e., a minimum revolution speed can be setas low as possible. Alternatively, a voltage for revolution at a lowestspeed (lowest speed setting voltage) is set large in place of thedetected temperature voltage. Then, when starting the single-phase motorin a stopped state (including restarting), the width of the pulse signalof the comparator is too narrow to obtain a necessary driving duty, andthus the motor cannot be started. Therefore, it is impossible to set lowa minimum value of a revolution speed of the single-phase motor.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a single-phase motordriving circuit comprises a controller for outputting a control signalto drive a single-phase motor in a period in which a saw-tooth voltageof a predetermined cycle is larger than a duty setting voltage based ona result of comparing a detected temperature voltage changed based on atemperature detected by a temperature detecting device with a startingduty setting voltage increased with time while a voltage smaller thanthe detected temperature voltage at the time of starting thesingle-phase motor is set as an initial value. The controller sets theduty setting voltage as the starting duty setting voltage when theresult of the comparison shows that the starting duty setting voltage issmaller than the detected temperature voltage, and sets the duty settingvoltage as the detected temperature voltage when the starting dutysetting voltage is larger than the detected temperature voltage.

Features and objects of the present invention other than the above willbecome clear by reading the description of the present specificationwith reference to the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings wherein:

FIG. 1 is a circuit diagram showing a single-phase motor and its drivingcircuit according to an embodiment of the present invention;

FIG. 2 is a circuit diagram showing a specific constitutional example ofa 3-input comparator of the embodiment of the invention; and

FIG. 3 is a waveform chart showing a main signal in the single-phasemotor driving circuit of the embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

At least the following matters will be made clear by the description inthe present specification and the description of the accompanyingdrawings.

Brief Description of the Disclosure

A single-phase motor driving circuit according to an embodiment includesa controller for outputting a control signal to drive a single-phasemotor in a period in which a saw-tooth voltage of a predetermined cycleis larger than a duty setting voltage based on a result of comparing adetected temperature voltage changed based on a temperature detected bya temperature detecting device with a starting duty setting voltageincreased with time while a voltage smaller than the detectedtemperature voltage at the time of starting the single-phase motor isset as an initial value.

The controller sets the duty setting voltage as the starting dutysetting voltage when the result of the comparison shows that thestarting duty setting voltage is smaller than the detected temperaturevoltage, and the duty setting voltage as the detected temperaturevoltage when the starting duty setting voltage is larger than thedetected temperature voltage.

The controller may be constructed in an integrated circuit.

The single-phase motor driving circuit may further include a section forgenerating the detected temperature voltage, a section for generatingthe starting duty setting voltage, and a section for generating thesaw-tooth voltage.

The initial value of the starting duty setting voltage is preferablysmaller than a minimum value of the saw-tooth voltage.

The section for generating the starting duty setting voltage has atleast a capacitor, and a power source is connected to one end of thecapacitor, while a switching device is connected to the end. Theswitching device is turned ON to discharge electricity from thecapacitor when the single-phase motor is stopped, and the switchingdevice is turned OFF to start charging the capacitor from the powersource when the single-phase motor is started. A transitional risingvoltage which appears at the end of the capacitor in the charging stepmay be the starting duty setting voltage.

The single-phase motor driving circuit can further include a logicsection for generating a driving signal based on the control signal andoutputting the generated driving signal the single-phase motor.

A single-phase motor driving method according to an embodiment includesthe steps of: outputting a control signal to drive a single-phase motorin a period in which a saw-tooth voltage of a predetermined cycle islarger than a duty setting voltage based on a result of comparing adetected temperature voltage changed based on a temperature detected bya temperature detecting device with a starting duty setting voltageincreased with time while a voltage smaller than the detectedtemperature voltage at the time of starting the single-phase motor isset as an initial value; setting the duty setting voltage as thestarting duty setting voltage when the result of the comparison showsthat the starting duty setting voltage is smaller than the detectedtemperature voltage; and setting the duty setting voltage as thedetected temperature voltage when the starting duty setting voltage islarger than the detected temperature voltage.

Overall Constitution of Single-Phase Motor Driving Circuit

Description will be made for an overall constitution of the single-phasemotor driving circuit of the embodiment by referring to a circuit blockdiagram of FIG. 1. In the embodiment, it is assumed that thesingle-phase motor driving circuit is integrated, and peripheral circuitdevices such as a single-phase coil are externally connected to thecircuit.

As shown in FIG. 1, NPN type bipolar transistors 102, 104 receivedriving signals A, D to supply driving currents in a right direction inthe drawing (‘a’ direction) of a single-phase coil 106. Thus, acollector emitter path of the bipolar transistor 102, the single-phasecoil 106 and a collector emitter path of the bipolar transistor 104 areserially connected between a power source VCC and a ground VSS.Similarly, NPN type bipolar transistors 108, 110 receive driving signalsC, B to supply driving currents in a left direction in the drawing (‘b’direction) of the single-phase coil 106. Thus, a collector emitter pathof the bipolar transistor 108, the single-phase coil 106 and a collectoremitter path of the bipolar transistor 110 are serially connectedbetween the power source VCC and the ground VSS.

The bipolar transistors 102, 104 and the bipolar transistors 108, 110are complementarily turned ON/OFF to properly change the directions ofthe driving currents of the single-phase coil 106, whereby thesingle-phase motor is revolved. A regenerative diode 112 regenerates adriving current when the direction of the driving current of thesingle-phase coil 106 is changed from the a direction to the bdirection, and it is connected in parallel to the collector emitter pathof the bipolar transistor 110. Similarly, a regenerative diode 114regenerates a driving current when the direction of the driving currentof the single-phase coil 106 is changed from the b direction to the adirection, and it is connected in parallel to the collector emitter pathof the bipolar transistor 104.

Thus, when the bipolar transistors 102, 104 and the bipolar transistors108, 110 are complimentarily turned ON/OFF to revolve the single-phasemotor, a driving duty of the revolution is decided in accordance with acontrol signal outputted from a driving duty deciding comparator CMP.That is, a detected temperature voltage VTH, a starting duty settingvoltage RMI, and a triangular wave signal (saw-tooth voltage) PWM areinputted to the driving duty deciding comparator CMP. The control signaloutputted from the driving duty deciding comparator CMP is a pulsesignal in which 25 kHz is a basic frequency. In other words, pulse widthmodulation (PWM) control is carried out for ON/OFF-driving of thebipolar transistors 102, 104 and the bipolar transistors 108, 110.

Therefore, as shown in FIG. 1, in a period in which the driving currentis supplied to the single-phase coil 106 in the direction of an arrow a,the bipolar transistor 104 is always maintained in an ON state, whilethe bipolar transistor 102 is turned ON/OFF based on the basic frequencyof 25 kHz. Conversely, in a period in which the driving current issupplied to the single-phase coil 106 in the direction of an arrow b,the bipolar transistor 110 is always maintained in an ON state, whilethe bipolar transistor 108 is turned ON/OFF based on the basic frequencyof 25 kHz.

Then, according to the present invention, the pulse signal from thedriving duty deciding comparator CMP fluctuates based on the basicfrequency of 25 kHz in accordance with changes in the detectedtemperature voltage VTH and the starting duty setting voltage RMI asdescribed later. This fluctuation changes the ON/OFF operations of thebipolar transistors 102, 104 and the bipolar transistors 108, 110 tocontrol the driving duty of the single-phase motor.

A hall device 116 is fixed to a predetermined position opposite a magneton the rotor side of the single-phase motor, and biased by a constantvoltage. The hall device 116 outputs a sine-wave signal in accordancewith a revolution position of the single-phase motor, i.e., a change inthe magnetic pole of the opposite rotor side.

A comparator circuit 118 has hysteresis characteristics for preventingchattering, and uses the sine-wave signal from the hall device 116 as arectangular wave signal. The rectangular wave signal is a commutationsignal which is a basis for switching the driving current of thesingle-phase coil 106 to any one of the a and b directions.

A revolution stop detection circuit 122 includes a capacitor 124, aconstant current source 126, an NPN type bipolar transistor 128, acomparator circuit 130 and a reference voltage VREF, and detectsrevolution or stop of the single-phase motor. Herein, the capacitor 124and the constant current source 126 constitute a charging circuit, andthe capacitor 124 and the bipolar transistor 128 constitute adischarging circuit. As a result, a charge/discharge voltage having asaw-tooth shape appears on an ungrounded side of the capacitor 124. A−(inverting input) terminal of the comparator circuit 130 is connected tothe reference voltage VREF, while a + (non-inverting input) terminal isconnected to the ungrounded side of the capacitor 124. That is, thecomparator circuit 130 compares the voltage on the ungrounded side ofthe capacitor 124 with the reference voltage VREF in size to output adetection signal which becomes “L” during the revolution of thesingle-phase motor, and “H” during the stop of the single-phase motor.

The output from the revolution stop detection circuit 122 is supplied toa starting duty setting voltage generation circuit (section forgenerating starting duty setting voltage) 200. This starting dutysetting voltage generation circuit 200 includes an NPN type bipolartransistor (switching device) Tr1, and a charge/discharge circuitconnected to the outside. The charge/discharge circuit includes a powersource Vcc, a capacitor C1 and voltage-dividing resistors R1, R2. Thecapacitor C1 is connected in parallel to the voltage-dividing resistorR1. A parallel circuit constituted of the capacitor C1 and thevoltage-dividing resistor R1, and the voltage-dividing resistor R2 areserially connected between the power source Vcc and the ground Vss. Withrespect to the bipolar transistor Tr1, an output voltage of therevolution stop detection circuit 122 is applied to its base, and itsemitter is grounded. A collector of the bipolar transistor Tr1 isconnected to a connection point between the parallel circuit,constituted of the capacitor C1 and the voltage-dividing resistor R1,and the voltage-dividing resistor R2. A starting duty setting voltageRMI is generated at the connection point. That is, a voltage betweenterminals of the capacitor C1 becomes a starting duty setting voltageRMI.

A thermistor Rs and a resistor R3 are serially connected between thepower source Vcc and the ground Vss to constitute a detected temperaturevoltage generation circuit (section for generating detected temperaturevoltage) 300. The thermistor Rs is attached to a housing of a fan inorder to detect an ambient temperature of the fan driven by thesingle-phase motor. A detected temperature voltage VTH which reflectsthe ambient temperature of the fan is generated at a connection pointbetween the thermistor Rs and the resistor R3. The thermistor Rs has anegative temperature coefficient, and the detected temperature voltageVTH is reduced when a temperature rises in the housing.

The output voltage VTH from the detected temperature voltage generationcircuit 300, the output voltage RMI from the starting duty settingvoltage generation circuit 200, and a triangular wave signal (saw-toothvoltage) PWM from a PWM circuit (section for generating saw-toothvoltage) are inputted to a driving duty deciding comparator CMP. Thisdriving duty deciding comparator (controller for outputting controlsignal to drive the single-phase motor) CMP includes a 3-differential(3-input) comparator. The PWM circuit (PWM in FIG. 1) outputs atriangular wave signal PWM as a PWM control signal in order to control arevolution speed of the single-phase motor.

As a specific constitutional example, as shown in a circuit diagram ofFIG. 2, the driving duty deciding comparator CMP includes a constantcurrent source, three PNP type bipolar transistors Tr10, Tr20, Tr30, oneNPN type bipolar transistor Tr40, a bias resistor R10, and a resistorR20. Emitters of the three bipolar transistors Tr10, Tr20, Tr30 areconnected to the constant current source. Collectors of the two bipolarresistors Tr10, Tr20 are grounded. The bias resistor R10 is connectedbetween a base and an emitter of the bipolar transistor Tr40, and aconnection point between this emitter and the bias resistor R10 isgrounded. A collector of the bipolar transistor Tr30 is connected to thebase of the bipolar transistor Tr40. The power source Vcc is connectedthrough the resistor R20 to a collector of the bipolar transistor Tr40.

In the driving duty deciding comparator CMP of the aforementionedconstitution, a detected temperature voltage VTH is applied to the baseof the bipolar transistor Tr10. A starting duty setting voltage RMI isapplied to the base of the bipolar transistor Tr20. A triangular wavesignal PWM is applied to the base of the bipolar transistor Tr30. Anoutput signal of the driving duty deciding comparator CMP appears as adriving duty control signal at the collector of the bipolar transistorTr40.

A logic circuit (logic section for generating a driving signal based oncontrol signal and outputting the generated driving signal to thesingle-phase motor) 132 executes signal processing based on outputs ofthe driving duty deciding comparator CMP and the comparator circuit 118.As a result, as described above, the logic circuit 132 outputs drivingsignals A, B, C, D to complementarily turn ON/OFF the bipolartransistors 102, 104 and the bipolar transistors 108, 110.

The circuit shown in FIG. 1 can be constituted of one integrated circuitexcluding the detected temperature voltage generation circuit 300.However, a constitution can be employed in which the portion of theaforementioned charge/discharge circuit (or only capacitor C1) of thestarting duty setting voltage generation circuit 200 is externallyattached.

Operation of Single-Phase Motor Driving Circuit

Description will be made for a characteristic operation of thesingle-phase motor driving circuit of the embodiment by referring to thecircuit diagram of FIG. 2 and the waveform chart of FIG. 3.

First, a control principle of a driving duty will be described byreferring to the waveform chart of FIG. 3. The waveform chart of FIG. 3is a conceptual view for easy explanation of the operation.

As shown in periods T0, T0′ of FIG. 3, a starting duty setting voltageRMI is increased with time at the time of starting the single-phasemotor which is a driving target, while a voltage smaller than a detectedtemperature voltage VTH is set as an initial value. That is, at theinitial time of starting (at the time of supplying power), a voltage(starting duty setting voltage RMI) between the terminals of thecapacitor C1 is zero. At the time of restarting, since a voltage on theungrounded side of the capacitor 124 is larger than the referencevoltage VREF, an output of the comparator circuit 130 becomes “H” toturn ON the bipolar transistor Tr1. As a result, a voltage (startingduty setting voltage RMI) between the terminals of the capacitor C1 isalso zero at the time of restarting.

The driving duty deciding comparator CMP compares the starting dutysetting voltage RMI with the detected temperature voltage VTH. Any ofthe starting duty setting voltage RMI or the detected temperaturevoltage VTH, which is found to be smaller as a result of the comparison,is set as a duty setting voltage. Then, as shown in the waveform chartof the comparator output in FIG. 3, the driving duty deciding comparatorCMP outputs a control signal of “H” only in a period in which thetriangular wave signal PWM is larger than the duty setting voltage.

That is, the starting duty setting voltage RMI is smaller than thedetected temperature voltage VTH during the starting of the single-phasemotor which is the driving target (periods T0, T0′ of FIG. 3). In thiscase, the driving duty deciding comparator CMP sets the starting dutysetting voltage RMI as a duty setting voltage. As a result, the drivingduty deciding comparator CMP outputs a control signal only in the periodin which the triangular wave signal PWM is larger than the starting dutysetting voltage RMI. Thus, compared with the conventional case in whicha lowest speed setting voltage is a duty setting voltage, it is possibleto increase a pulse width of the output of the driving duty decidingcomparator CMP. Therefore, at the time of starting the single-phasemotor, a driving duty can be set large to enable starting of thesingle-phase motor.

By setting an initial value (about 0 V in FIG. 3) of the starting dutysetting voltage RMI smaller than a minimum value of the triangular wavesignal, it is possible to surely start the single-phase motor by amaximum driving duty at the initial starting stage of the period T0 inFIG. 3.

Subsequently, the starting duty setting voltage RMI is graduallyincreased as the time passes. Then, as shown in a period T1 of FIG. 3,when the starting duty setting voltage RMI becomes a certain maximumvalue (lowest speed setting voltage in FIG. 3), the driving dutydeciding comparator CMP outputs a control signal of a narrowest pulsewidth in order to revolve the single-phase motor at a lowest revolutionspeed for the time of a low temperature.

The certain maximum value of the starting duty setting voltage RMI,i.e., the lowest speed setting voltage, can be properly set by changinga capacity value of the capacitor C1 in the starting duty settingvoltage generation circuit 200 of FIG. 1. As it is not involved in thedriving duty at the time of starting, the lowest speed setting voltagecan be set lower compared with the conventional case to enable a furtherreduction in a lowest revolution speed.

A temperature rise caused by the revolution of the single-phase motor isaccompanied by a gradual reduction in the detected temperature voltageVTH. Thus, when the starting duty setting voltage RMI passes through across point in FIG. 3 to become larger than the detected temperaturevoltage VTH (period T2, PWM control variable speed area in FIG. 3), thedriving duty deciding comparator CMP switches the duty setting voltageto the detected temperature voltage VTH. As a result, the driving dutydeciding comparator CMP outputs a control signal only in a period inwhich the triangular wave signal PWM is larger than the detectedtemperature voltage VTH. In other words, the single-phase motor isrevolved by a driving duty in accordance with the detected temperatureVTH of the thermistor Rs.

When the temperature rise caused by the revolution of the single-phasemotor progresses, and the detected temperature VTH of the thermistor Rsbecomes high (period T3, full speed area in FIG. 3), a control signaloutputted from the driving duty deciding comparator CMP becomes fulldriving of a duty 100%.

Next, description will be made for an operation of the circuit devicewhich realizes the foregoing driving duty control principle.

Description will be made of a process from the starting (or restarting)of the single-phase motor in a stopped state to the driving by thedriving duty in accordance with the detected temperature VTH of thethermistor Rs (periods T0, T1 in FIG. 3). As shown in FIG. 2, the outputvoltage (detected temperature voltage) VTH and the triangular wavesignal PWM have been respectively inputted from the detected temperaturevoltage generation circuit 300 and the PWM to the driving duty decidingcomparator CMP. In addition, the output voltage (starting duty settingvoltage) RMI has been inputted from the starting duty setting voltagegeneration circuit 200 to the comparator CMP.

As shown in FIG. 3, the starting duty setting voltage RMI is changedwith time at the time of starting. That is, a detection signal outputtedfrom the revolution stop detection circuit 122 of FIG. 1 is changed from“H” indicating the stop of the single-phase motor to “L” indicating therevolution of the single-phase motor. This detection signal “L” isapplied to the base of the bipolar transistor Tr1 of the starting dutysetting voltage generation circuit 200 in FIG. 1, and the bipolartransistor Tr1 is switched from an ON state to an OFF state.

That is, first, in the stopped state of the single-phase motor, as shownin FIG. 1, the capacitor C1 is in a discharged state since the bipolartransistor Tr1 is in the ON state. Thus, for the starting duty settingvoltage RMI inputted to the comparator CMP, only a low voltage (about 0V in FIG. 3) of an ON resistance portion of the bipolar transistor Tr1is generated. This voltage V1 is an initial value of the starting dutysetting voltage.

In the process from the stopped state to a starting state throughapplication of the triangular wave signal PWM to the base of the bipolartransistor Tr30, the detected temperature voltage VTH is applied to thebase of the bipolar transistor Tr10, and the starting duty settingvoltage RMI is applied to the base of the bipolar transistor Tr20. Atthis time, as shown in FIG. 3, the starting duty setting voltage RMI ismuch lower than the detected temperature voltage VTH. Thus, the bipolartransistor Tr10 to the base of which the detected temperature voltageVTH is applied is turned OFF, while the bipolar transistor Tr20 to thebase of which the starting duty setting voltage RMI is applied is turnedON. Then, the bipolar transistor Tr30 to the base of which thetriangular wave signal PWM is applied is turned OFF in the period inwhich the triangular wave signal PWM is larger than the starting dutysetting voltage RMI. As a result, the bipolar transistor Tr40 at thecollector of which the output of the comparator CMP appears outputs asignal of “H” in the period in which the triangular wave signal PWM islarger than the starting duty setting voltage RMI. As shown in thewaveform chart of FIG. 3, at the initial starting stage of the periodT0, the starting duty setting voltage RMI is smaller than the minimumvalue of the triangular wave signal PWM. Thus, the single-phase motorstarts driving by a full duty.

At the same time, the switching of the bipolar transistor Tr1 in FIG. 1to the OFF state is accompanied by starting of charging of the capacitorC1 which has been in the discharged state. Then, as the voltage betweenthe terminals of the capacitor C1 transitionally rises, the startingduty setting voltage RMI inputted to the comparator CMP is graduallyincreased. A degree of this increase can be set based on a capacity ofthe capacitor C1 and values of the voltage-dividing resistors R1, R2which constitute the charge/discharge circuit. In the periods T0, T0′,T1 in the process of this increase, the starting duty setting voltageRMI is still smaller than the detected temperature voltage VTH. Thus,the bipolar transistor Tr10 to the base of which the detectedtemperature voltage VTH is applied is kept OFF, while the bipolartransistor Tr20 to the base of which the starting duty setting voltageRMI is applied is kept ON. Then, the bipolar transistor Tr30 to the baseof which the triangular wave signal PWM is applied is turned OFF in theperiod in which the triangular wave signal PWM is larger than thestarting duty setting voltage RMI, and turned ON in the period in whichthe triangular wave signal PWM is smaller than the starting duty settingvoltage RMI. As a result, the bipolar transistor Tr40 at the collectorof which the output of the comparator CMP appears outputs a signal of“H” in the period in which the triangular wave signal PWM is larger thanthe starting duty setting voltage RMI, and outputs a signal of “L” inthe period in which the triangular wave signal PWM is smaller than thestarting duty setting voltage RMI. Thus, the comparator CMP outputs apulse signal in which the period of “H” becomes shorter as the startingduty setting voltage RMI rises.

Then, as shown in the periods T2, T3 of FIG. 3, when the starting dutysetting voltage RMI becomes larger than the detected temperature voltageVTH after the completion of charge of the capacitor C1, the bipolartransistor Tr10 to the base of which the detected temperature voltageVTH is applied is kept ON, while the bipolar transistor Tr20 to the baseof which the starting duty setting voltage RMI is applied is turned OFF.

Then, the bipolar transistor Tr30 to the base of which the triangularwave signal PWM is applied is turned OFF in the period in which thetriangular wave signal PWM is larger than the detected temperaturevoltage VTH, and turned ON in the period in which the triangular wavesignal PWM is smaller than the detected temperature voltage VTH. As aresult, the bipolar transistor Tr40 at the collector of which the outputof the comparator CMP appears outputs a signal of “H” in the period inwhich the triangular wave signal PWM is larger than the detectedtemperature voltage VTH, and outputs a signal of “L” in the period inwhich the triangular wave signal PWM is smaller than the detectedtemperature voltage VTH. Thus, the comparator CMP outputs a pulse signalin accordance with the detected temperature voltage VTH of thethermistor Rs.

If the rotor of the single-phase motor is locked, since no dischargepulse is generated from the logic circuit 132, the voltage between theterminals of the capacitor 124 becomes larger than the reference voltageVREF. As a result, the bipolar transistor Tr1 is turned ON, and thevoltage between the terminals of the capacitor C1 (staring duty settingvoltage RMI) becomes zero. Thus, at the time of restarting, thesingle-phase motor starts driving by a full duty similarly to the caseat the time of supplying power.

When starting the single-phase motor, the single-phase motor can bestarted (restarted) based on the starting duty setting voltage. Thus,irrespective of a driving duty at the time of starting, a minimum valueof the driving duty can be set as small as possible in order to revolvethe single-phase motor at a lowest speed after the starting.

Although the preferred embodiment of the present invention has beendescribed in detail, it should be understood that various changes,substitutions and alterations can be made therein without departing fromspirit and scope of the inventions as defined by the appended claims.

1. A single-phase motor driving circuit comprising: a controller foroutputting a control signal to drive a single-phase motor in a period inwhich a saw-tooth voltage of a predetermined cycle is larger than a dutysetting voltage based on a result of comparing a detected temperaturevoltage changed based on a temperature detected by a temperaturedetecting device with a starting duty setting voltage increased withtime while a voltage smaller than said detected temperature voltage atthe time of starting said single-phase motor is set as an initial value,wherein said controller sets said duty setting voltage as said startingduty setting voltage when said result of the comparison shows that saidstarting duty setting voltage is smaller than said detected temperaturevoltage, and said controller sets said duty setting voltage as saiddetected temperature voltage when said starting duty setting voltage islarger than said detected temperature voltage.
 2. The single-phase motordriving circuit according to claim 1, wherein said controller isconstructed in an integrated circuit.
 3. The single-phase motor drivingcircuit according to claim 1, further comprising: a section forgenerating said detected temperature voltage; a section for generatingsaid starting duty setting voltage; and a section for generating saidsaw-tooth voltage.
 4. The single-phase motor driving circuit accordingto claim 1, wherein said initial value of said starting duty settingvoltage is smaller than a minimum value of said saw-tooth voltage. 5.The single-phase motor driving circuit according to claim 2, furthercomprising: a section for generating said detected temperature voltage;a section for generating said starting duty setting voltage; and asection for generating said saw-tooth voltage.
 6. The single-phase motordriving circuit according to claim 2, wherein said initial value of saidstarting duty setting voltage is smaller than a minimum value of saidsaw-tooth voltage.
 7. The single-phase motor driving circuit accordingto claim 3, wherein said initial value of said starting duty settingvoltage is smaller than a minimum value of said saw-tooth voltage. 8.The single-phase motor driving circuit according to any one of claims 3through 7, wherein said section for generating the starting duty settingvoltage includes at least a capacitor, a power source is connected toone end of said capacitor and a switching device is connected to theend, said switching device is turned ON to discharge electricity fromsaid capacitor when said single-phase motor is stopped, said switchingdevice is turned OFF to start charging said capacitor from said powersource when said single-phase motor is started, and a transitionalrising voltage, which appears in said end of said capacitor in saidcharging step, is said starting duty setting voltage.
 9. Thesingle-phase motor driving circuit according to any one of claims 1through 7, further comprising a logic section for generating a drivingsignal based on said control signal and outputting the generated drivingsignal to said single-phase motor.
 10. The single-phase motor drivingcircuit according to claim 8, further comprising a logic section forgenerating a driving signal based on said control signal and outputtingthe generated driving signal to said single-phase motor.
 11. Asingle-phase motor driving method comprising the steps of: outputting acontrol signal to drive a single-phase motor in a period in which asaw-tooth voltage of a predetermined cycle is larger than a duty settingvoltage based on a result of comparing a detected temperature voltagechanged based on a temperature detected by a temperature detectingdevice with a starting duty setting voltage increased with time while avoltage smaller than said detected temperature voltage at the time ofstarting said single-phase motor is set as an initial value; settingsaid duty setting voltage as said starting duty setting voltage whensaid result of the comparison shows that said starting duty settingvoltage is smaller than said detected temperature voltage; and settingsaid duty setting voltage as said detected temperature voltage when saidstarting duty setting voltage is larger than said detected temperaturevoltage.